A method as mentioned before is known from the publication of H. P. Kleinknecht and H. Meier in Journal of Electrochemical Society, Vol. 125, No. 5, May 1978, pp. 798 to 803. This document discloses a method wherein the optical grating is composed of openings in the form of grooves provided in a photoresist layer formed on the surface of a dielectric layer of silica (SiO.sub.2) or of silicon nitride (Si.sub.3 N.sub.4). The photoresist layer otherwise serves as a mask to obtain by reactive ion etching the etching of patterns into the dielectric layer, which is in turn formed on the surface of a semiconductor substrate of silicon (Si). The grating is intended to control in situ the depth of etching of the dielectric layer by reactive ion plasma etching. For this purpose, the optical grating is illuminated by a laser beam having a wavelength of 632.8 nm, which is modulated and directed substantially perpendicularly to the optical grating arranged in the ion etching structure. The control is effected by the analysis of the light diffracted by the grating. For this purpose, the light intensity originating from the grating is controlled by a photoelectric diode.
The control of the step of etching the dielectric layers is obtained by the interpretation by means of models of the quantitative analysis of the oscillations of the intensity diffracted by the grating. In order to obtain the diffraction conditions, the periodicity of the grating can vary from 3 to 10.2 .mu.m, the latter value being preferred. The order of diffraction 1 is only considered.
The problem solved by the aforementioned method is the detection of the instant at which the dielectric layer is completely etched and the semiconductor substrate is exposed in order to avoid that the substrate is etched. By measurement of the diffraction intensity, this instant is known by the fact that it coincides with an abrupt diminution of the diffracted intensity, which is then maintained at a low and constant level.
The aforementioned document also indicates that the control of the step of etching the dielectric layers could also be effected by measuring the intensity of the specular reflection. However, in this case, the laser beam would have to be directed to a region of the layer devoid of any irregularity on a surface equally wide as the diameter of the laser beam, which condition can be obtained in practice only with difficulty during the manufacture of integrated circuits on semiconductor substrates. Especially in this case, an optical grating could not be used.
Such a technique utilizes the diffraction of the light. The interpretation is simple in the ideal case of a grating for which only the depth of the etching lines varies, that is to say a model with only 1 parameter. The modellization is very rapidly complexer in the realistic case in which the assembly of the geometry of the grating is modified during the plasma attack. Several parameters act upon the diffracted intensity, of which the contributions practically cannot be separated.
It is especially necessary that not only the thickness of the etched dielectric layers, but also the depth etched into one or several subjacent semiconductor material layers can be measured, as well as the roughness of the etching edges or the roughness of the bottom of the opening etched into the semiconductor material or materials.
These problems are solved by means of the method according to the invention.